In order to support the communication requirements of high-speed data transmission applications at a bit rates of 25 Gpb, optical links are used when links through an electrical wire have a too low bandwidth. When using an optical link for transmitting an electrical signal from a first electronic component to a second electronic component, the electrical signal is first converted into an optical signal, then the optical signal is coupled into an optical fiber through an optical transmitter, and the optical signal is then transmitted to the second electronic component through the optical fiber. At the second electronic component, the optical signal is received by an optical receiver and converted back into an electrical signal. Then the converted electrical signal is further processed in the second electronic component.
FIG. 1 shows a conventional transceiver 100 for converting an electric signal into an optical signal and vice-versa. The transceiver 100 includes a dielectric, non-conductive substrate (not shown in FIG. 1), a ground plane 101 placed on the dielectric non-conductive substrate, an array of four drivers 102, an array of four laser diodes 103 such as vertical cavity surface emitting lasers (VCSEL) connected to respective outputs of the drivers 102, an array of four photodiodes 104 such as positive intrinsic negative diodes (PINs), and an array of four transimpedance amplifiers 105 (TIAs) connected to respective outputs of the photodiodes of the array of photodiodes 104. The array of drivers 102, the array of laser diodes 103, the array of photodiodes 104 and the array of TIAs 105 are all placed on the same dielectric non-conductive substrate and surrounded by the ground plane 101. Additionally, the array of photodiodes 104 is placed in the interior of an opening provided in the ground plane 101, so as to isolate the array of photodiodes 104 from the ground plane 101.
Each driver of the driver array 102 receives at its input (not shown in FIG. 1) an electric signal from, for instance, a motherboard of a computer (not shown in FIG. 1), converts the received electrical signal into a single ended electrical signal, and outputs this through output terminals 110 and 111 to inputs of a VCSEL of the VCSEL array 103. The output terminal 111 of a driver of the driver array 102 is connected through a ground line 113 to an input of a respective VCSEL of the VCSEL array 103. The output terminal 110 of a driver of the driver array 102 is connected through a signal line 112 to another input of the respective VCSEL. Each VCSEL of the VCSEL array 103 converts the single ended electrical signal received at its inputs to an optical signal, and outputs this to an optical fiber.
Each photodiode 114 of the array of photodiodes 104 receives an optical signal from an optical fiber (not shown in FIG. 1), converts the received optical signal into a single ended electrical signal, and outputs this through its anode 117 and its cathode 116 to the inputs of a respective TIA. The anode 117 of a photodiode of the array of photodiodes 104 is connected through a signal line 118 to an input of the respective TIA, and the cathode 116 of a photodiode of the array of photodiodes 104 is connected through a ground line 119 to another input of the respective TIA.
As the signal lines 112 connecting outputs of the driver array 102 with respective inputs of the VCSEL array 103 (Driver-VCSEL channels) and the signal lines connecting anodes 114 of the photodiode array 104 with respective inputs of the TIA array 105 (in the following denoted as PIN-TIA channels) are of single-end type, so single-end-type crosstalk occurs among these lines. The single-end type crosstalk occurs where there is a transfer of signal power from one or a plurality of signal lines (aggressor lines) to another signal line (victim line) through the common ground plane 101. At the victim line, crosstalk overlays with the signal carried by the victim line, thereby degrading its signal quality. Crosstalk can occur not only among Driver-VCSEL channels and among PIN-TIA channels, but also among Driver-VCSEL and PIN-TIA channels, if the array of photodiodes 104 is isolated from ground by the opening 115 provided in the ground plane 101.
The opening 115 formed in the ground plane 101 and surrounding the photodiode array 104 has a rectangular shape and a size of approximately 1 mm×0.4 mm. The size of the opening is mainly determined by the dimensions of the photodiode array 104 and cannot be reduced arbitrarily. At high frequencies, the opening 105 acts as a slot resonator whose fundamental frequency is determined by the geometric dimension of the opening 105. A rectangular opening having a size of 1 mm×0.4 mm has a resonant frequency of about 38 GHz. This is about 3×12.5 GHz, wherein 12.5 GHz is the fundamental frequency of the 25 Gbps data transmission. As the opening 115 acts as a slot resonator whose fundamental frequency is almost a multiple of the fundamental frequency of the 25 Gbps data transmission, the opening 115 attracts signal current output by drivers of the driver array 102. This resonant effect, which occurs at approximately 38 GHz, promotes crosstalk from Driver-VCSEL channels to PIN-TIA channels. Particularly, PIN-TIA channels that are placed near to the driver array 102 are affected by crosstalk coming from Driver-VCSEL channels.
FIG. 2 shows the principle s-parameter coupling coefficient 201 corresponding to the single end type crosstalk from the driver outputs of the driver array 102 to the input of the TIA closest to the driver array 102. This is obtained by simulation for the transceiver 100 having all Driver-VCSEL channels (aggressors) activated. The principle s-parameter coupling coefficient 201 exhibits a broad resonance 202 at 38 GHz due to the resonance effect of the opening 115. This resonance 202 induces crosstalk in the TIA input closest to the driver array 102. This crosstalk generates more than 6 ps or 0.15 UI Jitter in the EYE diagram for 25 Gbps, assuming a bit error rate (BER) of 10−12. This becomes evident from FIG. 3, which shows the EYE diagram corresponding to 25 Gbps, for the PIN-TIA channel closest to the driver array 102. The EYE diagram in FIG. 3 has been obtained by simulation under the conditions that the transceiver 100 has all four aggressor channels activated and the ratio between driver output and TIA input current is equal to 100, which corresponds to about −8 dBm optical receiver sensitivity. The outline 301 corresponds to a bit error rate of 10−12.
The resonant behavior of the opening 115 also promotes crosstalk among PIN-TIA channels. This increases with increasing number of photodiodes included in the array of photodiodes 104. When the photodiode array 104 includes 8 or 12 channels, the crosstalk level among PIN-TIA channels can reach an intolerable value.
For a photodiode array including twelve channels, the opening provided in the ground plane and surrounding the photodiode array exhibits a resonance at about 15 GHz. This resonance is close to the fundamental frequency (12.5 GHz) of the 25 Gbps data transmission and thus also promotes crosstalk, thereby degrading signal quality of the PIN-TIA channels.
Crosstalk in the PIN-TIA channels severely degrades the signal quality of the signals input to the TIA array 105, especially of those input to TIAs that are close to the driver array 102.